Why does milk turn sour

"Lactic acid is produced as a result of microbial activity which gives the milk its sour taste"


Milk is rich in carbohydrates, fats and protiens which in turn creates an ideal enviroment for spoilage by microbes. Lactobacillus a harmless bacteria uses lactose in milk to for enery and gives lactic acid as a byproduct. And this lactic acid gives the sour taste to milk. The production of lactic acid causes the protien casein contained in milk to turn into curd. 

However when milk is pasteurized, it kills most of the lactobacillus bacteria contained in milk, thus milk can last longer in a refrigerator. Lactobacillus is a very useful bacteria and it is because of this bacteria's existense that so many different kinds of diary products such as cheese, yougurt etc exist.


[1] http://en.wikipedia.org/wiki/Soured_milk

[2] http://www.thehindu.com/todays-paper/tp-features/tp-sci-tech-and-agri/milk-turning-sour/article3225464.ece

[3] http://www.howitworksdaily.com/why-does-milk-turn-sour/